Various types of infrared sensors have been developed and are used for a wide variety of thermal imaging applications. MEMS-based microbolometers generally include a substrate having thereon a focal plane array (FPA), the FPA including a plurality of detector elements that each correspond to a respective pixel in the image produced by the sensor. Accordingly, individual detector elements may be generally referred to herein as pixels. An infrared-transparent cover, or lid structure, is placed over the FPA and attached to the substrate to provide a vacuum environment in which the MEMS-based microbolometer can operate. These lids are often coated with an anti-reflective coating to reduce the reflective properties and increase the infrared transmission properties of the lid. The substrate contains an integrated circuit which is electrically coupled to the detector elements, and which is commonly known as a read out integrated circuit (ROIC) and which is used to integrate the signal from each pixel and multiplex the signals off the chip with appropriate signal conditioning and processing. Each pixel includes a membrane which is suspended at a location spaced above the top surface of the substrate, in order to facilitate thermal isolation. The membrane includes a thermally sensitive material, such as amorphous silicon (a-Si) or vanadium oxide (VOx). The membrane also includes two electrodes, which are each coupled to the thermally sensitive material, and which are also coupled to the ROIC in the substrate. As the temperature of the thermally sensitive material varies, the resistance of the thermally sensitive material also varies, and the ROIC in the substrate can determine the amount of thermal energy which has been received at a pixel by sensing the corresponding resistance change of that pixel.
As the microbolometers have been made more sensitive to incoming electromagnetic radiation, they have also become more sensitive to effects of self-heating, which causes a change in the intensity output from the pixels of the array. Changes in the intensity outputs tend to be non-uniform across the many detector elements of the array, causing different pixels receiving the same input radiation to produce different outputs, and contributing to noise in the image.
One approach to non-uniformity correction (NUC) in uncooled microbolometer arrays is to periodically shutter the FPA (or shutter the lens that focuses incident electromagnetic radiation onto the FPA) for a few seconds to allow for a non-uniformity correction of the image to be calculated. Some conventional uncooled infrared microbolometers operating at ambient temperature and without the use of active temperature stabilization use infrared optically blind reference pixels that do not absorb incident infrared radiation to provide reference measurements that can be used for NUC. These infrared optically blind reference pixels are used to determine ambient temperature of the focal plane which is required in the calibration of the focal plane array over the operating temperature of the focal plane array. This involves implementation of a gain and offset correction algorithm at any given temperature (sensed by the reference pixels) to the active detector elements in order to correct the image for ambient temperature drift effects, e.g., in an imaging focal plane array. The reference pixels are generally placed on the sides of the rows or columns of the FPA to compensate for non-uniformity by continuously normalizing the row-to-row or column-to-column non-uniformity. However, non-uniformities also occur within the rows and columns, requiring the image to be corrected using a shutter. Another method of non-uniformity correction is known as scene-based NUC, which requires the viewed scene to be changing and may involve complex image processing.